I don’t know how many of you were Jericho fans (CBS television show that has reportedly been permanently cancelled), but from the time I first watched it, I fell in love with how they developed the characters, and the parallels with good fiction.
It’s important to have fully fleshed out characters and story, and yet start in the middle of the action. But by only revealing the characters’ backstory a bit at a time, you leave some mystery for the reader to follow. If the characters are good, readers are fascinated as they get to know them little by little.
Read Reserve Backstory in Fiction for ideas and literary examples by Toni Morrison and Anna Quindlen, and then give it a try.